(a) Field of the Invention
The invention relates to a novel knee laxity evaluator (KLE) system.
The invention also relates to a motion module/digitizer combination which can be used in the KLE, or which can be used independently or in other systems. More specifically, the invention relates to such a combination which can measure, in three dimensional space, and relative to the position of a first point or body or co-ordinate system, position or motion of a second point or body, as well as position or motion of the second point or body relative to a thrid, fourth, fifth . . . nth points, or positions of the second body, or combinations thereof.
(b) Description of the Prior Art
Currently, the practice of measuring knee laxity is limited to a subjective evaluation by a physician of relative displacements at the knee. Through such an examination, damage to ligaments could be ascertained as a function of excess laxity or joint movement during passive loading by the physician. The limitations of this technique are: (a) a high level of subjectiviey; (b) no quantitative or reproducible results; (c) no knowledge of applied forces; and (d) there are complicated motions which cannot be evaluated by human feel alone and hence there is important information being lost.
In accordance with the present invention, a KLE includes a motion module, that is, a module for measuring, in three dimensional space, movement of a point or body relative to a fixed point or body. Modules of this type are known in the art as is illustrated, for example, in U.S. Pat. No. 3,944,798, Eaton, Mar. 16, 1976, U.S. Pat. No. 4,057,806, Furnadjiev et al, Nov. 8, 1977, and U.S. Pat. No. 4,205,308, Haley et al, May 27, 1980.
Electrical and electronic digitizers are also known in the art. For example, a two dimensional digitizer is illustrated in U.S. Pat. No. 3,956,588, Whetstone et al, May 11, 1976.
However, there are no teachings in the art for combining the first systems, usually referred to as motion modules, and digitizers, whereby it is possible to measure the position or motion of a second point or body relative to the position of a first point or body and also relative to third, fourth, fifth . . . nth points or positions of the second body or combinations thereof.
Also in accordance with the present invention a KLE includes a force dynamometer, that is, a device for measuring external forces and moments applied to a body.
Force dynamometers are known in the art and are often referred to as multi-axis load cells, for example in U.S. Pat. No. 4,092,854, Henry et al, Apr. 20, 1977. Multi-axis load cells which have been previously described suffer from one or more of the following drawbacks:
1. Poor resolution PA1 2. Hysteresis PA1 3. Use of greater than the minimum six transducers inherently required to resolve the force and moment components PA1 4. Cross influencing between the various transducers.
With the exception of the drawback of using more than the minimum six transducers required the other problems listed generally result from the construction of the load cells. The problem has been to provide a structurally stable load cell while at the same time permitting one unique set of six transducer outputs for a given externally applied force and/or moment. In the above-mentioned reference, for example, the arrangement of flexure arms and flexure beams reduces, but does not eliminate cross influencing of combined load signals. If for example, a combined load were applied resulting in the flexion of a flexure arm about an axis through its center, parallel to the primary Z-axis, this load would be picked up not only in the six transducers but some would be "lost" by being absorbed in the chassis, through the flexure arms, and thus not transmitted to the transducers.